I have a Nexus instance with Maven 2 repositories hosted on a non-Internet-connected corporate development domain. I need to use the maven-jaxb-schemagen plugin from Sun, however, it was only available in Maven 1 format (along with many other Maven 1 dependencies, so this is not a one-off edge case).

I made a new Maven 1 hosted repository in Nexus (maven1), then made a Maven1-to-Maven2 virtual repository as well (maven1as2). Because some of the needed artifacts are snapshots, the policy for maven1 (and by extension maven1as2) is Snapshot.

I imported all of the Maven 1 artifacts through the filesystem to this repository's overridden local storage location.

I then added maven1as2 to my Public Repositories group. Some of the reference I have found says that Nexus will not allow you to browse or search a Maven1 repository. I have had mixed results. If I select Public Repositories or maven1as2 in Nexus, I can go to Browse Storage and drill down to com/sun/tools/jxc/maven2/maven-jaxb-schemagen-plugin/1.3-SNAPSHOT/ and view the maven-jaxb-schemagen-plugin-1.3-SNAPSHOT.pom file and its checksum. However, I cannot view the JAR artifact that should be there and is on the filesystem.

I learned that most of the time rebuilding metadata of the repositories helps to solve such problems.
I find it annoying how often this is necessary.

Select the repository

In the "Browse Storage" tab open the context menu of the root node (it sometimes is hidden behind the Firefox default context menu - pressing Esc helps in this case and brings the Nexus context menu into view)

Select rebuild metadata

Expire cache and reindex may help too for some cases.
Most of the time these steps didn't solve the problem at the first try but at last they solved it each time though.